1. Field of the Invention
This invention concerns high gain Raman fiber optic amplifiers, and optical fibers suitable for use in such amplifiers.
2. Discussion of the Known Art
Optical fibers having a high concentration of Germanium in their core, for example, fibers wherein the refractive index of the core is more than 0.020 above that of the surrounding cladding (xcex94n greater than 0.020) and with a small effective area (less than 20 xcexcm2), typically achieve good gain when used as media for Raman amplification. For example, such fibers are used as dispersion compensating fibers (DCFs) in commercially available dispersion compensating modules (DCMs). See, e.g., U.S. patent application Ser. No. 10/099,820 filed Mar. 16, 2002, entitled xe2x80x9cRaman Amplified Dispersion Compensating Modulesxe2x80x9d, which application is assigned to the assignee of the present application and invention.
Amplifiers that use fibers exhibiting high Raman gain are generally viewed as a means for achieving a high and flat gain characteristic in multiple wavelength intervals, limited only by the associated pump configuration. When comparing Raman amplifiers and Erbium doped fiber amplifiers (EDFAs), the lengths of gain fiber required for the respective amplifiers differ greatly. For example, only a few tenths of a meter of fiber is needed for an EDFA while several kilometers of fiber are typically required in a Raman amplifier. Because of this, a typical Raman amplifier is likely to introduce some significant dispersion and dispersion slope when placed in a communication system.
While an EDFA may introduce little dispersion to a communication system, the amplifier only provides gain in wavelength intervals of about 1528 to 1565 nm and 1570 to 1615 nm. A Raman amplifier, however, is capable of operation over many wavelength intervals, being limited only by the available pump configuration. Specifically, the gain region of a Raman amplifier is at a wavelength about 100 nm longer than the pump wavelength. Thus, if gain is needed at 1500 nm, setting a pump wavelength at about 1400 nm would enable a Raman amplifier to provide the required gain.
It is also generally known that a xe2x80x9czeroxe2x80x9d net dispersion may be achieved by combining fibers having equal positive and negative dispersions, respectively, at the wavelengths of concern. Such technique is used in modern optical communication systems wherein a positive dispersion of a transmission fiber is compensated by a fiber with negative dispersion in a slope matching dispersion compensating module (DCM). But a fiber pair having a neutral overall dispersion comprising a typical transmission fiber and a slope matching DCM, would not be an effective Raman amplifier since the length of the transmission fiber must be many times greater than the length of the module fiber. Also, typical transmission fibers are poor media for Raman amplification since they have low modal Raman gain coefficients (0.4 to 0.7 1W/km) and effective areas above 50 xcexcm2. For common transmission fibers the required length would be about 7 times greater than the length of the module DCF. For dispersion shifted fibers such as e.g., Truewave(copyright) fiber available from OFS Fitel, the required length would be about 25 times greater than that of the DCF.
A DCM available from OFS Fitel employs a dispersion compensating fiber known as RightWave(copyright) DK fiber. The RightWave DK fiber exhibits a negative dispersion slope which compensates about 65% of the dispersion slope of a conventional single mode fiber, with dispersion values available as low as xe2x88x922040 ps/nm at 1550 nm.
Dispersion tolerances of modern optical communication systems are extremely narrow. Accordingly, a Raman fiber optic amplifier which obtains high gain with substantially zero net dispersion over a wide bandwidth when placed in a communication system, would be highly desirable.
According to the invention, an optical fiber suitable for use as a gain fiber in a Raman fiber optic amplifier includes a generally cylindrical core, an outer cladding, and a refractive index profile with respect to the outer cladding. The core has a diameter of between 3 and 6 microns (xcexcm) and a difference (xcex94n) between the index of the core and the outer cladding is between 0.015 and 0.035. The index profile includes a trench region adjacent the circumference of the core, and the trench region has a width of between 1 and 4 xcexcm and a relative index of between xe2x88x920.005 and xe2x88x920.015.
According to another aspect of the invention, a Raman fiber optic amplifier includes a first fiber having a signal input end and an output end opposite the signal input end, a second fiber having a signal output end and an input end opposite the signal output end, wherein the input end is spliced or coupled to the output end of the first fiber, and a pump light source coupled to the first and the second fibers in such manner as to achieve Raman amplification with respect to a light signal applied to the signal input end of the first fiber and output from the signal output end of the second fiber. Lengths of the first and the second fibers are determined so that the fibers exhibit dispersions of substantially equal magnitude and opposite sign at a wavelength of the light signal. The fiber having a positive dispersion includes a generally cylindrical core, an outer cladding, and a refractive index profile with respect to the outer cladding. The core has a diameter of between 3 and 6 microns (xcexcm) and a xcex94n of between 0.015 and 0.035. The index profile includes a trench region adjacent the circumference of the core, and the trench region has a width of between 1 and 4 xcexcm and a xcex94n of between xe2x88x920.005 and xe2x88x920.015.
For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims.